Late positivities in unusual situations

Abstract

Language processing leaves a large number of differentand variable traces in the EEG. Two ERP effects how-ever are generally seen as highly reliable. Manipulationsof syntactic structure elicit a late positivity, startinground 600 ms after the critical event and lasting untilabout 800–900 ms. These positivities are elicited by allsorts of grammaticality violations, but also by sentenceambiguity and by a high degree of sentence complexity.Manipulations in the meaning domain on the other handelicit a negativity, peaking at about 400 ms after the crit-ical event. This is the N400 effect, perhaps the mostintensively studied of all ERP effects, and generally seenas an index of semantic integration, in that it reflects theease with which a word is integrated into its context, beit another word, a sentence or a piece of discourse. How-ever, this reassuring picture of a P600 effect reflectingsyntactic and an N400 effect reflecting semantic process-ing, has been brutally disturbed by a number of studiesin which semantic violations failed to elicit an N400effect but triggered a P600 effect instead. What are wegoing to make out of this confusing state of affairs? Itseems that there are two ways to go. On the one hand,we may need to think differently about the relationshipbetween syntax and semantics. This is the point of viewdefended by the authors of the two papers devoted tothis phenomenon in this issue. On the other hand, aswe will argue, the results may also indicate that we needto develop ways to think about the relationship betweenthe language system and the executive system.A first approach to the finding that semantic anomalieslead to a P600 rather than to an N400 effect is taken in thestudy by Kemmerer et al. Perhaps the semantic anomalieswhich elicited the ‘syntactic’ P600 effect in the previousstudies represent a violation of a grammatical-semanticconstraint, because they involve rearranged thematic roles,as in at breakfast the eggs would eat..., a type of sentenceemployed by Kuperberg et al. As Kemmerer et al. argue,there are other examples of such grammatical-semanticconstraints, in particular the order of descriptive prenomi-nal adjectives. Behavioural data indicate that languageusers find particular orders unacceptable. For instance,an attribute such as size should precede an attribute suchas colour. In support of this hypothesis, the authors foundthat sequences like a * brown big dog elicited a P600 effectand not an N400 effect. So the dividing lines between syn-tax and semantics are perhaps less clear than conventionallinguistic theories suggest. The fruitfulness of this approachalso became evident from earlier studies by these authorsreporting specific difficulties with grammatical-semanticconstraints on prenominal adjective order in a group ofbrain damaged patients. Unexpectedly, however, a P600effect was also observed after sequences involving twoadjectives contradicting each other, as in * a small bigdog. That both types of semantic anomaly elicit a P600effect can be explained by assuming that in both a processof restructuring takes place to make sense out of the anom-aly. By construing the sequence of the two adjectives andthe noun as a hierarchical—[Adj + [Adj + N]]—ratherthan as a flat—[Adj + Adj + N]—structure, the sequencescan be meaningfully interpreted as ‘a big dog that is brown’and ‘a small dog that is—relatively—big’ respectively.Think of the well-known movie in which Dustin Hoffmanplayed the role of Little Big Man, where little and bigcan describe the same person because ‘little’ indicates phys-ical size, whereas ‘big’ relates to character and courage.The process of restructuring makes it understandablewhy the ‘syntactic P600’ was evoked in both cases. Interest-ingly, both structures elicited not only a P600 effect but alsoa ‘reversed’ N400 effect in that the N400 amplitude wasreduced relative to the control condition. Only in the case